1. Field of the Invention
This invention relates to a method for press-molding a thermoplastic resin in which a plasticized resin is charged into a lower mold half of a mold in an open state, in a press-molding machine and the lower mold half is then clamped with an upper mold half to make compression molding.
An injection molding method is generally utilized for molding a product provided with a rib, a boss or an undercut portion, or a part having a thickened portion in a wide range. With this injection molding method, however, a material to be used as a molten resin injected into a closed mold cavity through a gate portion is limited to a resin having high fluidity, which gives little potential for using a resin having high molecular weight and high viscosity which have excellent physical properties. In addition, it is also difficult to mold a specific filler containing compound resin including various long fibers or metal powders and is also difficult to obtain a mold product which is thin. Moreover, in case of molding a product having a wide molding area, one molding cycle is protracted and the clamping force for clamping the molds is inevitably increased. These defects necessitate enlargement of the molding machine provided with multiple gates, thus requiring an increased cost for the equipment. The thus molded products are frequently flawed with a sink mark, warp or weld-mark, thus producing a problem in the quality of the product. In integral molding with a cloth having raised or soft foamed material, the application of high pressure in the injection molding operation may damage the characteristics of the products.
Conventional molding methods other than the injection molding method, such as vacuum and pressure molding method or a stamping molding method of a structual material also have a limitation in the shape of a product to be molded, which results in disadvantages of low productivity or high energy consumption.
These defects or disadvantages in the conventional molding methods are result from the fact that the plasticized resin is injected into the closed mold cavity through small nozzle means and, in order to obviate these defects, there is provided a compression molding method in which the plasticized resin is injected into the mold cavity formed by mold halves which are opened and the mold halves are then clamped under pressure. As a typical example of this method, "Resin Press-Molding Method Due to Direct Auto-charge of Resin" is disclosed in the Japanese Patent Post-Examination Publication No. 43012/83, which is generally called a stamping molding system mainly utilized for molding a large scale thin resin part for an automobile. According to this conventional method, the resin, plasticized and extruded by an extruder or extruding machine, has to be automatically measured so as to obtain an amount of plasticized resin corresponding in amount to one shot of the resin. The thus obtained resin is automatically charged into a lower mold of the press-molding machine while in an open state, and the molds are then clamped thereby spreading and press-molding the resin charged in the mold cavity. During this clamping operation, the molds are cooled and, after cooling, a molded product is taken out. In this operation, the measurement of the weight of the plasticized resin and the injection of the plasticized resin are performed by using an accumulating cylinder means and, accordingly, operations for stocking the melted and plasticized resin in the resin measuring chamber of the accumulating cylinder means and injecting the stocked resin into the mold cavity of the press molding machine have to be repeated intermittently to supply the resin into the lower mold. Accordingly, in this method, it is necessary to vary the stroke of the piston in the accumulating cylinder means every time so as to accord with the weight of the resin to be charged. These operations require much time for measuring and supplying the amount of resin for one charge and, hence, one molding cycle is protracted. The protracted molding cycle causes the resin in the mold to be cooled. Moreover, relatively complicated and large scale mechanisms such as hydraulic means for operating the piston of the accumulating cylinder means are required, and the cost of equipment therefore will be increased. In addition, many other movable members such as pivotable joint members are provided between the gate of the extruding machine and the accumulating cylinder means, so that there is a fear of leakage of the resin and a problem for the maintenance thereof.